1. Field of the Invention
This invention relates to television receivers, and more particularly, to a double-scanning non-interlace television receiver for displaying a received interlace television signal.
2. Description of the Prior Art
A television picture is a two-dimensional picture formed by scanning a one-dimensional time sequence signal. The time sequence signal is perceived as a picture which is continuous in time as well as space because of the time and space integrating functions of human vision. If the number of pictures displayed on a television screen per second is less than the threshold of human vision, the time integration effect is insufficient, and the picture is perceived as flickering. If the number of scanning lines comprising a television picture is below the threshold of human vision, the space integration effect of human vision is insufficient to produce an acceptable picture. The television picture is perceived as coarse, and the scanning lines are conspicuous.
Interlace scanning television systems, such as the NTSC system or the like, attempt to reduce flicker by increasing the number of pictures per second beyond the threshold of human vision. However, the television picture displayed with such an interlace scanning system is inferior to a television picture displayed with a non-interlace scanning system in which, for example, 525 lines are scanned. The interlace television picture is inferior because the space integration effect of human vision is not as effective in such an instance.
The space integration effect of human vision is not effective in the perception of a rapidly moving object displayed by an interlace scanning system, such as the NTSC system. If the object moves more quickly than the time required for the system to scan the second field comprising a frame, only the lines of the first field display the object. One half of the scanning lines, or 262.5 scanning lines, are used. The image, accordingly, becomes coarse. This effect is even more pronounced in a television receiver with a large screen. Although there are various factors which determine the quality of a television picture, one of the most important factors is the coarseness of each scanning line.
One solution to the problem of the coarseness of a scanning line is to double the number of scanning lines with an interlace system 1050 scanning lines. A second solution is to scan every line with a non-interlace system having 525 consecutively scanned lines. However, there are problems with displaying an interlace television signal having 525 scanning lines, such as the NTSC system, on receiver systems constructed in accord with such suggestions. In an interlace receiver system with 1050 scanning lines, a flicker occurs between the upper and lower portions of a scanning line. In a non-interlace receiver system with 525 scanning lines, a straight oblique line will be displayed as a zig-zag line because of the deviation between the position of the scanning line when it is repeated and the position when it is first displayed, since the same picture signal generates the two scanning lines.